Polymeric materials have been used for coating substrates in many industries. For example, in the automotive industry, these polymeric coatings, e.g., single coats or clear coats, are used to provide resistance to environmental acids for automotive products. Automotive topcoats represent one of the most demanding coating applications. These have to provide an aesthetic, long lasting appearance, and retain their gloss under the influence of moisture, UV radiation and temperature. Further, due to the acidity of rain, these automotive coatings must also be acid etch resistant.
To avoid environmental pollution and improve safety, reduced volatile organic solvent content (VOC), high solids content (HSC) automotive clear coats have been developed. The HSC coatings are predominantly based upon acrylic polymers. Over the past 10 years, the average molecular weights of the acrylic polymers have been lowered to achieve higher solids content and lower VOC, and the low molecular weight acrylic resins are cross-linked with an amino formaldehyde resin. As the molecular weight of the acrylic polymers is lowered, a higher level of the melamine cross-linker is required to achieve acceptable properties.
For example, the lower solids acrylic polymers which were previously used in automotive coatings contained about 20-25% melamine resin. At this level of melamine resin the acid etch resistance of the coating was acceptable. However, for high solids coatings, a melamine resin level of 30-45% was required to achieve sufficient crosslinking in a lower molecular weight resin, to provide solvent resistance and exterior durability. For a high molecular weight acrylic polymer with a molecular weight ("MW") of 100,000, a film with excellent mechanical properties will form without the need of a crosslinker. Thus, if the MW of a polymer is low, the polymer chain will need to be extended by adding a chain extension agent, i.e. a crosslinker. A further complication arises when the MW of a polymer is decreased. For example, acrylic polymer prepared by free radical polymerization have a random MW distribution. For a polymer with an average MW of 2000, there are high and low MW fractions. The low MW fractions are of concern. It is known that only a fraction of the monomer units contain functional groups such as hydroxyl groups, for chain extension. If 20% of the monomer units are functional and reactive with the melamine resins and a polymer chain contains only 5 monomer units, there will be on the average only one functional group per chain. Also, a certain proportion of the polymer chains will contain no functional groups. It has been found by experience that polymer chains without functional groups plasticize with a resultant decrease in exterior durability. Therefore, to assure the presence of sufficient functional groups on the low molecular weight polymer chains in a high solids acrylic polymer, the content of functional monomer has to be increased. As a result of this increase in the content of functional monomers, the content of the crosslinker must also be increased.
It has been found, however, that at higher levels of the melamine crosslinker the acid etch resistance of the polymer is reduced. Acid etch testing conducted on melamine resins crosslinked coatings show a clear relationship between acid etch resistance and melamine resin content. It is known that the ether linkage between the melamine resins and the acrylic polymer is acid catalyzed and, therefore, will hydrolyze under acid conditions. In contrast, the acrylic backbone itself consisting of carbon--carbon bonds is more resistant to acid attack.
The presently used HSC automotive coatings utilize hydroxyl function acrylic polymers having molecular weights of about 2000-5000 and a hydroxyl number of 150 to 200. Such high solid content acrylic polymers are commercially available, e.g., Acryloid QR-1120 available from Rohmand Haas or, Elveron 100 from Dupont. The melamine cross-linker is usually a fully alkylated hexamethylol melamine resin, such as, hexakismethoxymethylmelamine (HMMM), the oligomers thereof or a mixed ether melamine resin such as a methylated/butylated resin.
The composition of a typical mixed ether melamine resin is described in U.S. Pat. No. 4,374,164. The chemistry and reactivity of melamine resin is described in W. J. Blank, "Reaction Mechanism of Amino Resins," J. Coat. Techn., Vol. 51, No. 6567, pg. 61-70 September 1979; N. Albrecht and W. J. Blank, "The Use of Triazine Resins in High Solids Coatings", Proceedings of the Sixth International Conference in Organic Coatings and Technology, Athens, Greece, 1980; W. J. Blank, "Amino Resins in High Solids Coatings," J. Coat. Techn. Vol. 54; Nu 687; pg 26-41. The attack by acids on automotive coating is described in Alrich Schulz & Peter Trubiroha, "Simulated acid precipitations, Advances in the weathering of automotive finishes", Europcoat 9/1993, Pg 600-602. Formulations prepared from hydroxyl function acrylic polymers and HMMM are catalyzed with a strong sulfonic acid catalyst such as p-toluenesulfonic acid or dodecylbenzene sulfonic acid, dinonylnaphthalene disulfonic acid or the amine salts of these acids.
It has now been found that the low molecular weight acrylic/cross-linked melamine coatings of the prior art are sensitive to acid rain. As a result, when these HSC coatings are applied to surfaces that are exposed to typical industrial conditions, such as acid rain found in an industrial environment, the acid attacks the surface of the HSC coating. The acid rain causes leaching of the slightly basic melamine resins leading to a dull surface with the loss of gloss and eventually, pitting.
Two component acrylic/isocyanate coatings have been developed in an attempt to avoid this problem. However, because of the toxicity of the isocyanates and the short pot life of these coatings, they have not been accepted widely by the coating industry.
Thus, the objective of the present invention is to provide a polymeric coating that avoids the above mentioned problems. This objective have been achieved by polyurethane-polyol-melamine cross-linked polymers and coatings according to the present invention that have improved acid etch resistance.